In the forming section of a paper making machine, an aqueous stock is deposited onto the paper side surface of a moving forming fabric. The machine side surface of the forming fabric is in contact with the static fabric support elements in the forming section of the paper making machine. The forming fabric allows water to drain from the stock, and retains a proportion of the paper making solids in the stock on its surface to form an incipient paper web.
It has been found that the desirable characteristics for such fabrics are to a degree mutually incompatible, both in achieving an acceptable balance between the drainage, formation and retention properties of the fabric, and in other factors affecting the selection of weave patterns to achieve optimal properties for the paper side and the machine side of the forming fabric. The forming fabric must be capable of withstanding the mechanical and abrasive stresses imposed on it, which, in modern paper making machines where the forming fabric moves at a speed in excess of 70 kph, are substantial. To produce acceptable quality paper, the forming fabric should not cause marking, known as wire mark, on the sheet, and the percentage of the paper solids in the stock retained in the incipient paper web, known as first pass retention, should be as high as possible. In order to achieve a high first pass retention, the forming fabric must have good drainage characteristics and low water carrying properties, so that the removed water is readily transported through the fabric, without excessive drainage and loss of the paper solids. The fabric should also provide a uniform, planar support surface onto which the stock is delivered so that the paper making fibres are evenly supported by the component yarns of the paper side surface and the resulting sheet does not exhibit substantial variation in its fibre distribution and is stated to be “well-formed”. In addition, as a significant proportion of the fibres in the stock delivered onto the moving forming fabric tend to be oriented in the machine direction of the forming fabric, the fabric should provide adequate fibre support in the cross machine direction.
The need for a high drainage rate calls for a fabric with an open weave, but such a weave tends to cause wire mark and the incipient paper web tends to be formed somewhat in, rather than mostly on, the forming fabric paper side surface. A closely woven fabric provides better paper support and results in good first pass retention, and the paper is formed on, rather than somewhat in, the fabric, and is thus easier to release from the forming fabric. However, a closely woven fabric drains relatively poorly.
It has been found that improved drainage and fibre support characteristics can be achieved by ensuring that the frame openings in the paper side layer are substantially regular, and if the openings are rectangular, it is preferable that the longer side be oriented in the cross-machine direction. However, a related factor is the undesirable effect of forces which tend to induce adjacent pairs of weft yarns to move closer together, creating an asymmetry, known as “twinning”. This reduces alignment and registration of the paper side and machine side yarns, and the resulting different sized drainage passages adversely affect paper quality. Various methods have been suggested to resolve this problem.
Wilson, in U.S. Pat. No. 6,112,774, suggests that twinning results from excessive tension in machine direction yarns where those yarns interlace in the machine side layer with the cross-machine direction yarns in an “under 1, over 1, under 1” configuration, for example in the zig-zag -machine side layer weave pattern disclosed by Wright in U.S. Pat. No. 5,025,839. Wilson discloses a weave pattern which maintains the zig-zag pattern of Wright for the machine side layer, but suggests an arrangement of alternating machine direction yarns, in which the machine side layer interlacing points on adjacent machine direction yarns are offset by at least two cross machine direction yarns, as a means of reducing tension in the machine direction yarns.
Nevertheless, it has been found that twinning of paper side layer weft yarns continues to occur in weave patterns such as disclosed by Wilson in U.S. Pat. No. 6,112,774. Wilson further suggests, in WO 01/59208, that cross-machine direction yarns can be maintained in their original positions, i.e. that twinning can be reduced, by the use of suggested preferred materials for the manufacture of the machine direction yarns. These materials are said to encourage crimping, particularly where the machine direction yarns interweave with the cross machine direction yarns in an “over 1, under 1, over 1, under 1, over 1” configuration.
However, it has recently been found that twinning of paper side layer weft yarns adjacent to interweaving points in that layer can be avoided, or significantly reduced, in a double layer fabric, by using weave patterns which do not involve the close proximity of interweaving points on adjacent paper side layer warp yarns. This advantage is further enhanced where the weave pattern additionally does not involve the close proximity on a single warp yarn of the last one of a series of interweaving points in the paper side layer and an immediately adjacent interlacing point in the machine side layer. It has thus been found that the undesirable twinning effect can be significantly reduced by providing a weave pattern which maximizes the distance between interweaving points in the paper side layer on adjacent warp yarns, while increasing the internal float length of the warp yarns between the interweaving points on the paper side layer and the interlacing points on the machine side layer.
The degree of twinning of adjacent yams can be described in terms of the ratio of the difference of the distance (W) between one of a specific twinned pair of yarns and the adjacent non-twinned yarn, and the distance (T) between the twinned yam pair, to the distance W. This can be expressed as the ratio (W−T):W; or as a percentage (W−T)/W×100.
In a fabric with minimal twinning, this ratio would approach 0:1, or 0%; whereas in a highly twinned fabric, this ratio can be as high as 1:2, or 50%. It has been found for the fabrics of this invention that the ratio can be reduced to at least 0.1:1, or 10%, and more preferably can be reduced to between 0.05:1 and 0:1, or 5% to 0%.
The reduction of the twinning of the paper side weft yarns, together with the fact that all of the paper side layer weft yarns contribute to the support of the paper making fibres, leads to a greater regularity in the frame openings on the paper side surface of the paper side layer, and hence to a corresponding greater uniformity in the fibre support. It is well known that the overall frame size and the frame length in the machine direction are important parameters in the design of forming fabrics, and these topics are discussed by Helle, Torbjorn, “Fibre Web. Support of the Forming Wire”, Tappi Journal, Vol. 71, No. 1 (January 1988), pp. 112–117; and Johnson, D. B., “Retention and Drainage of Forming Fabrics”, Pulp & Paper Canada, Vol 85, pp. T167–172 (1984). The authors indicate that frame opening configurations have a significant influence on the drainage of the incipient paper web, and on the first pass retention characteristics of the forming fabric. It has been found that greater cross machine direction support is achieved by the use of designs having rectangular frame openings.
It has previously been considered that drainage problems in double layer forming fabrics result from the use of weave patterns requiring more than 8 sheds in the loom. For example, one aspect of such problems is noted in CPPA data sheet No. G18 (Rev. November 1994), at page 9. However, it has been found that suitable weave patterns can be created using designs requiring 9 sheds or more, with advantageous results, and without the expected disadvantages.
Consequently, it has been found that the lengths of the exposed floats of the machine side layer weft yarns on the machine side surface of the machine side layer in a double layer forming fabric can be increased. The resultant increased volume of weft material which is subjected to the abrasive forces of the machine can significantly extend the operational life of the forming fabric.
The present invention therefore seeks to provide a double layer forming fabric for a paper making machine, having increased resistance to machine side layer wear and abrasion. The invention provides for relatively long machine side layer weft yarn floats in the machine side surface, which are exposed to the abrasive wear experienced by the forming fabric as it is running in contact with the various stationary and moving elements in the forming section of the paper making machine. The invention also enables the use of larger diameter weft yarns than have previously been found feasible for use in double layer forming fabrics.
The present invention also seeks to provide a double layer forming fabric having an improved balance between water drainage and paper solids retention. The invention provides substantially rectangular paper side layer frame openings, having substantially the same width in the machine direction. The regular spacing of the yarns forming the perimeters of the frame openings provides a high degree of uniformity of support for the paper making fibres, so that the resulting sheet has a substantially uniform appearance and structure.
The present invention still further seeks to provide a double layer forming fabric having a weave pattern which produces a substantial reduction in the twinning of the paper side layer weft yarns.